Lipids

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  • Lipids
    • Triglycerides
      • Formed during condensation reactions between glycerol and fatty acids
        • bond form is known as an ester
        • fatty acids are long hydrocarbon chains (between 14&22 carbon atoms long)
          • fatty acids are commonly found in cells
        • Enzymes catalyse the condensation reaction by which triglycerides are formed
      • The hydrophobic properties of triglycerides are caused by hydrocarbon chain of the component fatty acids
        • Due to their hydrophobic properties, triglyceride molecules clump together (aggregate)
          • when they aggregate - they clump into huge globules in the presence of water, making them appear to be macromolecules
      • Quite large but relatively small when compared with starch
    • Unsaturated fatty acids
      • A fatty acid in which one or more pairs of adjacent carbon atoms in the hydrocarbon chain are linked by a double covalent bond ( C=C)
    • Saturated fatty acids
      • A fatty acid in which all the bonds between carbon atoms in the hydrocarbon chain are singe covalent bonds (C-C)
    • Roles of lipids in living organisms
      • Energy Store
        • When triglycerides are oxidised during respiration - energy is released
          • Some is lost to the environment as heat - most of it is used to make ATP
        • When fully respired, lipids release more than twice as much energy as do carbohydrates
          • Lipids form a more concentrated energy store than carbohydrates
        • A fat store is typical in animals that endure long unfavourable seasons
          • Oils are often a major energy store in seeds and fruit of plants
      • Waterproofing
        • Lipids are hydrophobic - they repel water
        • Mammals have oily secretions from the sebaceous glands on their skin
          • The oil acts as a water repellent - preventing fur and hair from becoming waterlogged when wet
      • Insulation
        • Lipids are poor conductors of both hear and hydrophilic ions
        • Triglycerides are stored in mammals as adipose tissue
          • Found under the skin known as subcutaneous fat
        • Fat reserves have a restricted blood supply - little body heat is distributed to the fat under the skin
        • Myelin is a lipid found in the surface membrane of cells
          • Wraps around the long fibres of nerve cells in animals
          • Myelin insulates the fibre preventing the passage of Na and K ions - essential for conduction of the nerve impulses
          • Nerve impulses travel faster when surrounded my myelin
    • Phospholipids
      • Has a similar chemical structure to a triglyceride
        • Except, in phospholipids one of the fatty acid groups is replaced by a phosphate group
      • Phospholipid molecules have 'heads'
        • The head is composed of a glycerol to which is attached to an ionised phosphate group
        • H bonds readily form between this phosphate group and H2O molecules - the head has hydrophilic properties
      • Phospholipids also consist of two long fatty acid residues - comprising hydrocarbon chains
        • These are known as 'tails' - hydrophobic properties
      • Phospholipids are partly hydrophobic and hydrophilic
      • A small quantity of phospholipid in contact with water will float - with the hydrocarbon tails exposed above the water
        • This forms a single layer (monolayer) of phospholipids
      • When more phospholipid is added the molecules arrange themselves as a bilayer
        • The hydrocarbon 'tails' face together away from the water and the hydrophilic heads in the water
      • Their hydrophobic & hydrophilic nature & their ability to form a bilayer is extremely important properties

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